1. Field of the Invention
This invention relates to measuring natural gamma radiation from a subsurface formation. In particular, the measuring is performed within a borehole.
2. Description of the Related Art
Geologic formations below the surface of the earth may contain reservoirs of oil and gas. Measuring properties of the geologic formations provides information that can be useful for locating the reservoirs of oil and gas. Typically, the oil and gas are retrieved by drilling boreholes into the subsurface of the earth. The boreholes also provide access to take measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the boreholes. In one embodiment, a “logging instrument” is lowered on the end of a wireline into the borehole. The logging instrument sends data via the wireline to the surface for recording. One type of measurement involves measuring naturally occurring gamma radiation (or gamma rays) from the geologic formations.
The geologic formations may include formation bedding planes. In a quest for oil and gas, it is important to know about the location and composition of the formation bedding planes. In particular, it is important to know about the formation bedding planes with a high degree of accuracy so that drilling time is not wasted.
Measuring naturally occurring gamma radiation is one way to determine characteristics of the formation bedding planes. A gamma radiation detector may be used as a component of the logging instrument to measure the naturally occurring radiation. Typically, the gamma radiation detector is cylindrically shaped and has a certain length. In typical embodiments, scintillator materials are used for gamma radiation detection.
A gamma ray entering the gamma radiation detector will cause an output of an electrical signal. Gamma rays may enter the gamma radiation detector from any angle. As long as a gamma ray enters the gamma radiation detector, the gamma radiation detector will output an electrical signal regardless of the angle of entry. As the gamma radiation detector moves along the borehole, gamma rays from the formation bedding planes will enter the gamma radiation detector and be detected.
One skilled in the art will recognize that various shapes and forms of gamma radiation detectors will provide various types of information about the formation bedding planes. As one may imagine, a cylindrically shaped gamma radiation detector may provide more opportunities for gamma rays to enter as the gamma radiation detector moves by the formation bedding planes. On the other hand, if the gamma radiation detector was, hypothetically, only a point, there would be fewer opportunities for gamma rays to enter as the gamma radiation detector moves by the formation bedding planes. A point gamma radiation detector can detect changes in radiation as the detector moves through the borehole with better spatial resolution than the cylindrically shaped gamma radiation detector. Less spatial resolution with cylindrically shaped gamma radiation detectors will result in less accurate knowledge of the formation bedding planes.
What are needed are an apparatus and a method for melting measurements of natural gamma radiation of a subsurface formation from a borehole with improved spatial resolution along the axis of the borehole.